Mechanosensitive non-equilibrium supramolecular polymerization in closed chemical systems

Chemical fuel-driven supramolecular systems have been developed showing out-of-equilibrium functions such as transient gelation and oscillations. However, these systems suffer from undesired waste accumulation and they function only in open systems. Herein, we report non-equilibrium supramolecular polymerizations in a closed system, which is built by viologens and pyranine in the presence of hydrazine hydrate. On shaking, the viologens are quickly oxidated by air followed by self-assembly of pyranine into micrometer-sized nanotubes. The self-assembled nanotubes disassemble spontaneously over time by the reduced agent, with nitrogen as the only waste product. Our mechanosensitive dissipative system can be extended to fabricate a chiral transient supramolecular helix by introducing chiral-charged small molecules. Moreover, we show that shaking induces transient fluorescence enhancement or quenching depending on substitution of viologens. Ultrasound is introduced as a specific shaking way to generate template-free reproducible patterns. Additionally, the shake-driven transient polymerization of amphiphilic naphthalenetetracarboxylic diimide serves as further evidence of the versatility of our mechanosensitive non-equilibrium system.


Preparation of buffer solutions:
Boric acid-potassium chloride-sodium hydroxide buffer solution (pH 8) was prepared by mixing 25 mL of boric acid-potassium chloride (0.2 M) with 4 mL of 0.1 M aqueous sodium hydroxide and then diluting the mixture to 100 mL with water. Ammonium chloride-ammonia buffer solution (pH 9.18) was prepared by mixing 0.1 mol/L ammonium chloride with 0.1 mol/L ammonia in a 2:1 ratio. Boric acid-potassium chloride-sodium hydroxide buffer solution (pH 10) was prepared by mixing 25 mL of boric acid-potassium chloride with 43.9 mL of 0.1 M aqueous sodium hydroxide and then dilute the mixture to 100 mL with water. Disodium hydrogen phosphate-sodium hydroxide buffer solution (pH 12) was prepared by mixing 50 mL of 0.05 M disodium hydrogen phosphate solution with 26.9 mL of 0.1 M aqueous sodium hydroxide solution and then diluting the mixture to 100 mL with water. Potassium chloride-sodium hydroxide buffer solution (pH 13) was prepared by mixing 25 mL of 0.2 M potassium chloride solution with 66 mL of 0.2 M aqueous sodium hydroxide solution and then diluting the mixture to 100 mL with water. To monitor the change in solution pH during the experiment, we used a pH meter to measure the pH of buffer solution (pH 12) containing C12-MV 2+ (3 mM) and PN (3 mM) in the presence of N2H4•H2O (5% v/v) over time. The results indicated that the pH of the buffer solution remained constant throughout the experiment.

The influence of viologen derivatives on supramolecular self-assembly.
Firstly, we investigated the influence of viologen derivatives on supramolecular selfassembly. The results showed that when amphiphilic molecules with long hydrophobic polar chains (C12-MV 2+ or C16-MV 2+ ) were dissolved in water containing PN, the selfassembly could be obtained due to charge transfer (CT) interaction and amphipathic interaction. Inhere, C12-MV 2+ with better water solubility was used for further studies.

The influence of pH on supramolecular self-assembly.
The influence of pH on supramolecular self-assembly was investigated by viscosity measurements. The results indicated that when the pH of the solutions was 12, the solutions containing C12-MV 2+ (10 mM) and PN (10 mM) exhibited the highest viscosity. Therefore, all subsequent measurements were carried out in the buffer (pH = 12). To support the maximum degree of aggregation of the assembly of equimolar C12-MV 2+ / PN samples, dynamic light scattering (DLs) was employed to determine the size of assembly. As shown in the Supplementary Figure 14, the results indicate that the assembly reaches its maximum diameter (108 nm) only when the ratio of incoming PN to C12-MV 2+ is 1.

The dissipative performance can be activated again by introducing fresh air in a closed system.
Supplementary Figure 18. Visualization of discoloration activated again by introducing fresh air in a closed system.

Gas phase changes in redox processes.
Firstly, we used gas chromatography to monitor the gas composition during the redox process, and the chromatogram showed a gradual decrease in oxygen and a progressive increase in nitrogen content over time (Supplementary Figure 19b). To better track the kinetic changes in gas composition, as shown in Supplementary Figure 19a, we employed an oxygen-nitrogen percentage detector to quantitatively monitor the gas phase change process. The results obtained from the detector were consistent with those obtained from gas chromatography measurements. Additionally, we utilized detectors with ppm-level sensitivity for nitric oxide and nitrogen dioxide to confirmed the absence of nitric oxide and nitrogen dioxide production.

Redox kinetics of C12-MV 2+ /PN in the presence of chiral molecules investigated by UV-vis measurements.
We investigated the time-dependent UV-vis spectra of shake-driven transient supramolecular helical structure in the presence of L-phenyllactic acid and Dphenyllactic acid, respectively. As shown in Supplementary Figure 24, we did not observe a significant red shift or blue shift in the spectra. We believed that a red or blue shift in the CD spectra was random, which might be the result of the unstable selfassembled structures under the non-equilibrium state.

The fluorescence of PN is absorbed by MV 2+ .
The quenching of PN fluorescence is due to the energy transfer between MV •+ and PN. The enhanced fluorescence induced by shake is due to the energy transfer between MV 2+ and PN was decreased.

Ultrasound-induced patterning.
We observed that oxygen entered the system with a fixed flow direction for directional oxidation during sonication, but after a little staying, instead of showing a flow line of oxygen entry, a pattern of snowflake spots appeared. We consider this because the viscosity of the system is so weak, which makes the flow line discontinuous. Therefore, a series of streamlined patterns were obtained by adding different levels of PEG (10 kDa) into the solution to increase the viscosity of the system. When the PEG concentration reached 5%, a complete heart-shaped streamline pattern was shown.

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Compared to MV 2+ , C12-MV 2+ itself has surface activity, which increases the viscosity of the system and decreases the surface tension of the system, so the direction of oxygen inflow can be observed without additional PEG addition. Because different concentrations of C12-MV 2+ have different effects on the surface tension of the system, we observed different sizes of the cardioid flow lines, which decrease with the decrease of surface tension.